Nonlinear change in refractive index and transmission coefficient of silicon at long-pulse,mJ-range, 1.54-μm excitation

An experimental study and theoretical modeling of the nonlinear changes in transmission coefficient and refractive index of mono-crystalline Silicon (Si) at long-pulse, mJ-range, single-beam Z-scan probing at 1.54 μm wavelength are reported. It is shown experimentally that at increasing pulse energy density the photo-induced darkening permanently increases in Si while its photo refraction properties demonstrate a more complicate character, being a product of various type nonlinearities. A theoretical analysis based on simple assumptions of a square-shape pulse in the time domain and Gaussian spatial distribution of the probe beam allows fitting of a whole of the experimentally measured open- and closed-aperture Z-scans through an account of the main contributions in the light-induced absorption and refractive index nonlinearities. These are revealed to originate from non-direct two-photon absorption and Kerr effect, induced absorption and dispersion of light-generated free carriers, and light-induced thermal lensing.     

Spectral emissivities of H2O vapor at 2900 K in the 1-9-μm region

Spectra produced by H₂O at 2900 K were measured, with various optical depths and the results compared with theoretical predictions. The H₂O vapor was produced as exhaust gas from burners fed by a mixture of natural gas and oxygen. Emission measurements were made near the burners, where the composition and temperature were shown to be uniform. The measured spectral emissivities were found to be in good agreement with predicted values, except for the 6.3-μm band.     

Analysis of the Isotopic Composition of Exhaled Air by the Diode Laser Spectroscopy Method in the 2-μm Region for Diagnosis of Helicobacter pylori-Associated Diseases

Optics and Spectroscopy - To diagnose digestive diseases associated with Helicobacter pylori infection, we applied laser spectroscopic analysis of the 13СО2/12СО2 isotope...     

The ro-vibrational analysis of the v4 fundamental band of CF3Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

The v₄ fundamental band of CF₃⁷⁹Br and CF₃⁸¹Br, present in natural isotopic abundance, was investigated in the 8.3-μm region by high-resolution infrared spectroscopic techniques. Tuneable diode laser spectra were recorded in the ranges 1202.5–1205.0 cm^?1, 1208.0–1210.1 cm^?1 and 1212.5–1214.5 cm^?1. The tuneable diode laser spectra were obtained at the reduced temperature of 200 K and in a free-jet expansion. The latter technique was used to reduce spectral congestion, achieving a rotational temperature of about 50 K, with a resolution up to 0.0008 cm^?1. A Fourier transform infrared spectrum covering the entire spectral region of the v₄ band, between 1190 and 1220 cm^?1, was recorded at 298 K with a resolution of 0.004 cm^?1. The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of v₄. In total, 4651 transitions were assigned to CF₃⁷⁹Br, 4047 to CF₃⁸¹Br, with J″_max? = K″_max?=80; of these, 3171 for CF₃⁷⁹Br and 2755 for CF₃⁸¹Br are from diode laser measurements. The data of each isotopologue were analysed using the model Hamiltonian for a degenerate vibrational state of a molecule of C_3v symmetry. The v₄ band of both the isotopologues resulted essentially unperturbed, but the Δl = Δk = ±2 l-resonance was found to be active within the v₄ = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of v₄ = 1 for CF₃⁷⁹Br and CF₃⁸¹Br were obtained. The bromine isotopic splitting amounts to 6.9 × 10^?3 cm^?1. In addition, the equilibrium geometry and the harmonic force field were calculated ab initio using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.     

Multispectrum fitting of line parameters for C2H2 in the 3.8-μm spectral region

Using FT spectra (Bruker IFS 120, unapodized FWHM resolution ≈ 0.001 cm⁻¹) of acetylene ¹²C₂H₂, absolute positions and intensities have been measured for about 250 lines between 2600 and 2800 cm⁻¹ in the and cold bands, and in the and hot bands. These measurements improve the accuracy of wavenumbers previously available and lead to individual line intensities for the first time in this spectral region. A multispectrum fitting procedure has been used to retrieve line parameters from five experimental spectra recorded at different pressures. The frequencies of the ν₃ band of ¹²C¹⁶O₂ allowed to perform an absolute wavenumber calibration. The accuracy of the amount of ¹²C₂H₂ in the sample has been checked using the cold band around 2100 cm⁻¹, and has been estimated to be around ±2%. The average absolute accuracy of the line parameters obtained in this work has then been estimated to be ±0.0002 cm⁻¹ for line positions, and ±5% for line intensities. For each studied band, the vibrational transition dipole moment squared value has been determined, as also empirical Herman-Wallis coefficients. A complete line list containing positions and intensities for the five strongest bands around 3.8 μm has been set up for atmospheric applications.     

AlGaN/GaN HEMTs on 4-Inch Silicon Substrates in the Presence of 2.7-μm -Thick Epilayers with the Maximum Off-State Breakdown Voltage of 500 V

We report on the high breakdown performance of AlGaN/GaN high electron mobility transistors （HEMTs） grown on 4-inch silicon substrates. The HEMT structure including three Al-content step-graded AlGaN transition layers has a total thickness of 2.7 μm. The HEMT with a gate width WG of 300 μm acquires a maximum off-state breakdown voltage （BV） of 550 V and a maximum drain current of 527 mA/mm at a gate voltage of 2 V. It is found that BV is improved with the increase of gate-drain distance LGD until it exceeds 8 μm and then BV is tended to saturation. While the maximum drain current drops continuously with the increase of LGD. The HEMT with a WG of 3 mm and a LGD of 8 μm obtains an off-state BV of 500 V. Its maximum leakage current is just 13 μA when the drain voltage is below 400 V. The device exhibits a maximum output current of 1 A with a maximum transconductance of 242 mS.     

Criteria for stabilizing a high spin d4 electronic configuration: Application to the synthesis and the mössbauer characterization of high spin iron (IV) in oxides

A Mössbauer study has been carried out on different oxides with the general formula (A, A)₂M_0.50Fe_0.50O₄ (A=Ca, Sr, Ba: A=La; M=Li, Mg, Zn, Ga, Ni). In all these phases with K₂NiF₄-type-derived structure the distortion of the (FeO₆) octahedra is sufficient to stabilize a high spin state for iron (IV) . An interpretation of the evolution of Mössbauer parameters is proposed, based on bond competition considerations.     

Experimental determination of the thermo-optic coefficient (dn /dT ) and the effective stimulated emission cross-section (σ e ) of an a -axis cut 1.-at. % doped Nd:GdVO4 crystal at 1.06 μm wavelength

In this paper, we report the experimental determination of the thermo-optical coefficient (dn/dT) and the effective stimulated emission cross-section (_e) at 1064 nm for an a-axis cut 1.3-at.% doped Nd:GdVO₄ crystal in a monolithic laser configuration. The pump-power-induced thermal lensing effect in the monolithic laser was used to determine these parameters. While measuring the dn/dT parameter, we also took into consideration the effect of pump-power-induced thermal expansion of the crystal. The dn/dT values obtained were 2.64-6 and 4.87×10^-6 K^-1, respectively, for directions parallel and perpendicular to the c-axis of the crystal. We show that the neglect of pump-power-induced thermal expansion of the crystal can overestimate the value of dn/dT by 30–50%. With the measured variation of the focal length of the thermal lens as a function of the absorbed pump power, we also computed the overlap integrals at the threshold pump power. These overlap integral values were used to estimate the product of the effective stimulated emission cross-section (_e) and the excited state lifetime () of the Nd:GdVO₄ crystal, which was found to be 1.476×10^-22 cm²s. With the reported values of for a 1.3-at.% doped Nd:GdVO₄ crystal, we estimated the value of _e to be in the range 14.76×10^-19 to 16.4×10^-19 cm². The value of the effective stimulated emission cross-section measured in this way was found to be around two times higher in magnitude than earlier reported values measured by spectroscopic methods. PACS 42.60.Lh; 42.60.Da; 42.60.By; 42.55.Rz     

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49-1.30 μm)

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by differential absorption spectroscopy between 6717 and 7351 cm⁻¹ (1.49-1.36 μm) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4% and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the ν₂+4ν₄ band near 6765 cm⁻¹. The reliability of the method based on the association of lines with coinciding centers in the 81 K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the ν₂+2ν₃ band near 7510 cm⁻¹. The line list for the whole icosad (6717-7655 cm⁻¹) consists of 12 865 transitions at 81 K.     

Determination of the low energy values of CH4 transitions in the 2ν3 region near 1.66 μm from absorption spectra at 296 and 81 K

The high resolution absorption spectra of ¹³CH₄ were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the ¹³CH₄ tetradecad dominated by the 2ν₃ overtone near 5988 cm⁻¹. Empirical line lists were constructed containing, respectively, 1629 ¹³CH₄ transitions detected at 81 K (5852-6124 cm⁻¹) and 3481 features (including 85 lines of ¹²CH₄) measured at room temperature (5850-6150 cm⁻¹); the smallest measured intensities are about 3 × 10⁻²⁶ and 4 × 10⁻²⁵ cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 ¹³CH₄ transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper ¹³CH₄ tetradecad.     

The ν1, ν4 and 3ν6 bands of methyl chloride in the 3.4-μm region: Line positions and intensities

Methyl chloride (CH₃Cl) is one of the most abundant chlorine-containing molecules in the atmosphere. For this reason a recent update was performed in HITRAN in the 640-2600 cm⁻¹ region based on the line parameters generated in Nikitin et al. [Nikitin A, Champion JP, Bürger H. J Mol Spectrosc 2005;230:174-84] with the intensities scaled to existing experimental data. CH₃Cl has a rather strong signature around 3000 cm⁻¹ which was used recently by the Atmospheric Chemistry Experiment (ACE) satellite mission to produce the first study of the global distribution of methyl chloride in the upper troposphere and stratosphere. However, it was mentioned that the CH₃Cl line positions and intensities spectroscopic parameters are of very low quality in this spectral region in the public access HITRAN or GEISA databases. We present a complete update of the line positions and line intensities for the ν₁, ν₄, 3ν₆ bands of CH₃ ³⁵Cl and CH₃ ³⁷Cl in the 3.4 μm region. For this task, Fourier transform spectra have been recorded at high resolution at the Laboratoire de Dynamique, Interactions et Réactivité in France. Measurements of line positions and line intensities have been retrieved for both isotopologues ¹²CH₃ ³⁵Cl and ¹²CH₃ ³⁷Cl in the ν₁, ν₄, 3ν₆ bands. The theoretical model accounts for the interactions coupling the (ν₁=1; ?=0), (ν₄=1; ?=±1) and (ν₆=3; ?=±1) energy levels, together with additional resonances involving several dark states.